The stories of brutality and destruction filtering out of the Jenin refugee camp have become increasingly ominous. While independent observers have been kept out - along with ambulances and UN blood supplies - the Israeli army has rampaged its way through the hillside shanty town, overwhelming desperate Palestinian resistance. Hundreds are reported killed, including many civilians. As in other West Bank towns and camps, reports of beatings and executions of prisoners abound, and Israel appears to be preparing the ground for evidence of atrocities. Meanwhile, across the Arab world - where TV news footage of Ariel Sharon's unleashing of state terror has been a good deal more graphic than what we have seen on our own screens - millions have demonstrated their fury at what is taking place, while their western-backed rulers have turned their guns on the streets, killing and injuring protesters from Bahrain to Alexandria.

This is where wars against terror end, with screaming children forced to drink sewage and piles of corpses being cleared by bulldozers. Yesterday's horrific suicide bomb attack on a bus in Haifa (from where many of the Jenin refugees fled or were expelled in 1948) has cruelly demonstrated the futility of the strategy pursued by Sharon and his government of national unity. The largest-scale Israeli offensive for two decades was supposed to root out the very terror networks that struck with deadly force yesterday. But such acts of desolate revenge are born of half a century of dispossession and powerlessness, and a civilian death count far higher than Israel has endured over the past 18 months. What alternative does the government have to defend its citizens in these circumstances, Israeli politicians demand. The answer is painfully obvious: withdraw from the territories it has lorded over since 1967 and redress the ethnic cleansing which underpinned the foundation of the state 19 years earlier.

Sharon has no intention of doing any such thing. Instead, he has plunged into a latterday version of France's war against the FLN insurrection in Algeria in the 1950s. Like Sharon's Israel, France unleashed its full might against bombers and gunmen, killing, torturing and imprisoning many thousands, crushing resistance in the casbahs with state terror. Yet after a lull, the rebellion reignited even more powerfully than before, and the French were forced to quit. Israelis usually have far fewer illusions about what is going on in their country than their western supporters. Michael Ben-Yair, Israel's attorney general in the mid-1990s, recently described the Palestinian intifada as a "war of national liberation", adding: "We enthusiastically chose to become a colonialist society, ignoring international treaties, expropriating lands, transferring settlers from Israel to the occupied territories, engaging in theft and finding justification for all these activities ... we established an apartheid regime".

But despite President Bush's much-vaunted public appeals to Sharon to begin a military pullback from the main Palestinian towns, the US - the one power in the world with the leverage over Israel to make it withdraw for good - shows no sign whatever of seriously reining in its long-term client state. On the contrary, the US administration, with the British government in ever-loyal echo, repeatedly expressed its "understanding" of Israel's attacks on Palestinian territory in the first phase of this invasion. Sharon's determination to destroy not just "terror networks" and the military infrastructure of the Palestinian Authority, but its civilian infrastructure as well - including educational and health institutions - has effectively had the green light from the US government. Both Sharon and Bush want to see the removal of the elected Palestinian leader, Yasser Arafat, even though his stature throughout the Arab world has grown dramatically as Israel has sought to humiliate him. Both appear to want the wider problem taken out of Palestinian hands and dealt with at a wider regional level. Nothing could have made the real US attitude clearer than the secretary of state Colin Powell's leisurely peregrinations across north Africa while Israeli forces have wreaked devastation in Jenin, Nablus and Bethlehem. To all intents and purposes, the destruction of the Palestinian Authority has been a policy signed off in Washington.

It can hardly be a surprise. US military and economic support for Israel - worth $70bn since 1979 - has after all been the linchpin of its imperial power in the Middle East since at least the 1960s. There is a widespread mythology, which at one end of the spectrum shades off into anti-semitic fantasies about global Jewish conspiracies, that US backing for Israel is largely the result of the effectiveness of political lobbying in Washington. In reality, it has been primarily driven by strategic interests in the world's most important oil region. Unlike the various autocratic Arab potentates the US and other western states lean on to keep the oil flowing and their populations in check, Israel is an utterly reliable ally with a proven military record against Arab armies. It was Israeli military prowess which broke the dangerous spell of Nasserism when it defeated the Arabs in the six day war. As a settler state in a hostile region, with a developed western political and economic system and dependent on US military and financial support, any Israeli move against US interests in the region is unthinkable. But while it is impossible to imagine Israelis electing an anti-western government, it would be a one-way bet in many Arab countries if their people were actually given a choice.

The pattern for the relationship was set by Britain as the dominant imperial power in the region in the early part of the last century. Sir Ronald Storrs, the first governor of Jerusalem under British rule in the 1920s, explained it as "forming for England a 'little loyal Jewish Ulster' in a sea of potentially hostile Arabism". A lifetime later, that is essentially the role played by Israel for the US and wider western interests today. It also helps explain the licence given to the Middle East's only nuclear-armed state to violate UN security council resolutions at will and why even the EU is unlikely to agree to the economic or military sanctions demanded yesterday by the European parliament. The closeness of the alliance does not, however, mean the US will not bring its client to heel if necessary. When US administrations have felt that Israeli behaviour was encroaching on vital US interests - as in 1956, when Israel seized the Suez canal in collusion with Britain and France, for example, or in the 1980s, when it tried to prevent the sale of Awacs surveillance aircraft to Saudi Arabia - they have been prepared to slap their ally down, regardless of its friends on Capitol Hill.

The paradox of Middle East peacemaking has long been that while the US is an open partisan of one side in the conflict, it is only through US intervention that a viable long-term settlement can be achieved. Bush's half-hearted attempt to strike a more even-handed public note over Sharon's onslaught in the West Bank this week is transparently the product of fears of growing unrest in the region - and the problems it is creating for US plans to settle accounts with Iraq. But the US will only move decisively if it feels its own interests are under threat.